Currently, the existing special launching carrier with under-bridge, such as the one disclosed in Chinese patent document No. ZL200420115944.4, may have the problem with erection for first span at the entrance and exit of a tunnel or erection for a short bridge (less than three spans) between two tunnels. It is due to the fact that the size overlaid is larger than the size of cross section of the tunnel and the horizontal height of a under-bridge differs from that of a bridge abutment when a special carrier for erecting beam and an under-bridge machine with platform are utilized for bridge erection, which makes it difficult to erect the first or the last span of an entire bridge, and also affect the stability of operation of the system and the safety of construction. This invention will provide a special launching carrier with variable position platform for solving the above problems.